The fire triangle is an illustrative model used to show what elements are needed to start and sustain a fire. The three elements that make up the triangle are heat, fuel, and an oxidizer (usually oxygen). While this model is at times a bit simplistic, it helps explain, in an easy-to-understand way, how most fires are started and sustained.
The starting point of any fire, and one corner of the triangle, is the fuel source. Without fuel, there is no substance to burn. Almost all substances on earth can serve as a fuel source if exposed to enough heat, but the most common are class A and B fuels, such as wood, paper, gasoline, and propane. In order for a fuel source to catch fire, two things must be present: enough oxygen to start and sustain the fire, and either a specific ignition source or a high enough ambient temperature to ignite the fuel.
Once a fire has started, as long as enough fuel and oxygen are present, the heat generated by the fuel source continues the combustion process in lieu of the original ignition source. The self-sustaining nature of fire is what makes it so dangerous and difficult to extinguish. When a fire gets too far out of control, sometimes extinguishing it is no longer a possibility and it must be left to burn out on its own. Due to the sheer size of forest fires, wildland firefighters must often cut down huge swathes of trees and clear away great amounts of underbrush in order to stop the fire from spreading, thereby depending on the fire to run out of fuel on its own.
In order to extinguish a burning fire, at least one of the elements of the fire triangle must be removed. There are different methods used for extinguishing different types of fires, but the most common way is to deprive the fire of oxygen with the use of a fire extinguisher. Most fire extinguishers spray a dry chemical, monoammonium phosphate, that melts on contact with the fuel source and cuts off oxygen by smothering it. The usually orange and red flames seen are not the source of a fire's heat and only a byproduct, which often causes confusion when someone is trying to put out a fire. Many people make the mistake of targeting the visible flames and not the actual fuel source when trying to put out a fire.